FIG. 1 illustrates a non-volatile static random access memory (nvSRAM) device with two power supplies. The primary supply is referred to herein as VCCX. Typical ranges of the primary supply voltage, for nvSRAM devices, are 2.7V-3.6V. An nvSRAM comprises volatile memory cells (RAM cells) each coupled with a corresponding non-volatile memory cell (nv cell). The nvSRAM performs various operations such as reads, writes, soft stores, and soft recalls as long as VCCX is ON.
The backup power supply is referred to herein as VCAP. VCAP is provided by one or more capacitors, typically “ultracapacitors” which have high energy capacity relative to conventional capacitors. When the external supply VCCX is switched OFF, or “browns out” (falls below a threshold voltage level), the nvSRAM operates from VCAP for a period of time. VCAP is typically pre-charged to a voltage Vcap>=Vccx, where Vcap is the voltage across the terminal of the backup capacitor, and Vccx is the primary power supply voltage. Typically, the only operation powered by VCAP is the transfer of the contents of the RAM cells to the nv cells of the nvSRAM (referred to herein as an “autostore” operation). When VCCX is powered up again, the contents of the nv cells are transferred back to the RAM cells by a process referred to as RECALL. The state of the RAM cells is thus preserved even in the absence of primary power.
In one implementation, VCAP is pre-charged to Vcap>=Vccx when VCCX is turned ON. An NMOS device with a diode configured between its source and drain may be used to charge VCAP (see FIG. 2). The gate of the NMOS device is boosted to 2*Vccx, so that there is no voltage drop across the NMOS device. Once VCAP is charged to Vccx, the nvSRAM is enabled for both volatile and non-volatile memory operations.
An autostore is initiated when the primary power supply VCCX becomes unavailable. FIG. 3 illustrates an example of how VCCX and VCAP may be operated during power up and power down. In this example, Vccx rises to about 2.7V. During the rise of Vccx at power-up, it passes a threshold value Vswp. Signal VCCX_HI is asserted at this point, indicating that Vccx has reached a sufficient high enough level to power a RECALL operation if so required. At Vswp, the NMOS gate which couples VCCX to VCAP turns ON. The memory device is enabled for reads and writes, for example approximately 20 ms (or after any pre-determined time delay) after power-on of VCCX.
When VCCX is switched OFF, meaning when VCCX dips below the threshold level of Vsws, VCCX_HI goes low, indicating primary power was lost. A request to do an autostore is initiated to the (internal micro-controller) memory controller. VCAP provides the energy to complete the autostore operation so that data is not lost before the memory powers down.